qa/common.webapi: improve logic in _wait_for_pmwebd_logfile()
qa/common.webapi: improve logic in _wait_for_pmwebd_logfile()

Rather than sleep 2 seconds, first call _wait_for_pmwebd() to
make sure the daemon starts and the socket is establised, then
a timed loop looking for the creation of the log file.

wuliming/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,wuliming/pcp,aeg-aeg/pcpfans,wuliming/pcp,adfernandes/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,adfernandes/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,wuliming/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,wuliming/pcp,aeg-aeg/pcpfans,adfernandes/pcp,adfernandes/pcp,wuliming/pcp